Signal generating apparatus and class-D amplifying apparatus

ABSTRACT

A signal generating apparatus includes: a data generator which generates a data series in which first, second, third and fourth data are arranged at a sampling period; a first signal generator which generates a first pulse-width modulation signal in which a pulse is arranged in a pulse period longer than the sampling period, time points of front and rear edges of the pulse being set in response to the first and second data; and a second signal generator which generates a second pulse-width modulation signal in which a pulse is arranged between the adjacent pulses of the first pulse-width modulation signal, time points of front and rear edges of the pulse of the second pulse-width modulation signal been set in response to the third and fourth data, respectively.

BACKGROUND OF THE INVENTION

The present invention is related to a technique for generating signals(will be referred to as “pulse-width modulation signals” hereinafter),the pulse widths of which have been set in response to a time sequenceof a plurality of data.

Signal generating apparatus (PWM modulation circuits) which generatepulse-width modulation signals from a time sequence of digital-formatteddata are suitably utilized in, for example, class-D amplifyingapparatuses. JP-A-2006-54815 discloses the following technical idea:That is, in a pulse-width modulation signal, time points of both a frontedge and a rear edge in each of pulses of the pulse-width modulationsignal are controlled in response to 2 pieces of data, so that a timeperiod (will be referred to as “pulse period” hereinafter) in which thepulses are arranged in the pulse-width modulation signal is made twotimes longer than a time length of a sampling period of data. Further,JP-A-2006-54815 describes such an arrangement that pulse-widthmodulation signals (w1(t) and w2(t)) of two systems which have anin-phase relation are generated in such a manner that 2 pieces of pulsesequivalent to a difference between both the pulse-width modulationsignals may have pulse widths in response to 2 pieces of data.

However, in any of the arrangements disclosed in JP-A-2006-54815, themodulating operations of 2 pieces of data are merely carried out everypulse period. As a result, there is such a restriction that the samplingperiod cannot be set to the time length shorter than a half of the pulseperiod.

SUMMARY OF THE INVENTION

While considering the above-described problem, the present invention hasan object to achieve such a technical idea that a sampling period is setto be a sufficiently short time with respect to a pulse period (namely,sampling frequency is sufficiently increased.

In order to solve the above-described problem, a signal generatingapparatus, according to the present invention, includes:

a data generator which generates a data series in which a plurality ofdata containing first data, second data, third data, and fourth data arearranged at a predetermined sampling period;

a first signal generator which generates a first pulse-width modulationsignal in which a pulse is arranged in a pulse period longer than thepredetermined sampling period, a time point of a front edge of the pulsebeing set in response to the first data, and a time point of a rear edgeof the pulse being set in response to the second data; and

a second signal generator which generates a second pulse-widthmodulation signal in which a pulse is arranged between the adjacentpulses of the first pulse-width modulation signal, a time point of afront edge of the pulse of the second pulse-width modulation signal beenset in response to the third data, and a time point of a rear edge ofthe pulse of the second pulse-width modulation signal been set inresponse to the fourth data.

In the above-described arrangement of the signal generating apparatus,the pulses of the first pulse-width modulation signal are defined basedupon the first data and the second data, and further, the pulses of thesecond pulse-width modulation signal are defined based upon the thirddata and the fourth data. As a result, the sampling period can be set tobe a sufficiently short time with respect to the pulse period (namely,sampling frequency of data series can be sufficiently increased).Accordingly, the electric power which is supplied to a load circuit canbe controlled in high resolution (namely, narrow stepping width) inresponse to the first pulse-width modulation signal and the secondpulse-width modulation signal. In addition, a dynamic range of theabove-described electric power can be easily secured (namely, S/N ratiocan be improved). It should be understood that in a preferred embodimentof the present invention, an interval among the respective pulsescontained in the first pulse-width modulation signal is equal to aninterval among the respective pulses contained in the second pulse-widthmodulation signal.

It should also be noted that a typical example of the above-describeddata generating generator corresponds to a noise shaping filter capableof suppressing a quantize noise. Alternatively, an oversampling circuitwhich over-samples a data series supplied from an upstream apparatus ina predetermined sampling period may also be employed as theabove-described data generator. In other words, the above-described datagenerator may be merely realized by any means capable of outputting adata series in which a plurality of data have been arranged in apredetermined sampling period. Therefore, there is no restriction in aconcrete structure of the data generator. Also, such an arrangementcapable of generating 3 series, or more series of pulse-width modulationsignals may be covered by the technical scope of the present invention.In the above-explained arrangement capable of generating 3 series, ormore series of the pulse-width modulation signals, one pulse-widthmodulation signal selected from 3 series, or more series of theabove-described pulse-width modulation signals is grasped as the firstpulse-width modulation signal of the present invention, and anotherpulse-width modulation signal selected therefrom is grasped as thesecond pulse-width modulation signal of the present invention.

In the signal generating apparatus according to a preferred embodimentof the present invention, the first signal generator generates the firstpulse-width modulation signal in such a manner that the larger thenumeral value of the first data, or the second data becomes, the widerthe pulse width thereof becomes; and the second signal generatorgenerates the second pulse-width modulation signal in such a manner thatthe larger the numeral value of the third data, or the fourth databecome, the narrower the pulse width thereof becomes. In accordance withthe above-described embodiment, it is possible to reduce such apossibility that the pulses of the first pulse-width modulation signalare overlapped with the pulses of the second pulse-width modulationsignal.

The signal generating apparatus related to the preferred embodiment ofthe present invention is provided with an adjusting unit (for example,adjusting unit 56 of FIG. 3) for changing a logic level of the firstpulse-width modulation signal and a logic level of the secondpulse-width modulation signal from each other in such a case that thepulse of the first pulse-width modulation signal is overlapped with thepulse of the second pulse-width modulation signal. In accordance withthe above-described embodiment, since the numeral values of thepreceding and succeeding data are largely changed, even in such a casethat the pulses of the first pulse-width modulation signal areoverlapped with the pulses of the second pulse-width modulation signal,the load circuit can be properly driven in response to the data series.

The signal generating apparatus related to the respective embodiments ofthe present invention may be suitably employed in a class-D amplifyingapparatus. A class-D amplifying apparatus, according to one preferredembodiment of the present invention, is equipped with the signalgenerating apparatus related to any one of the above-describedembodiments; a first driver (for example, driving unit 21 of FIG. 1) forcontrolling a supply of electric power with respect to a load circuit inresponse to the first pulse-width modulation signal; and a second driver(for example, driving unit 22 of FIG. 1) for controlling the supply ofthe electric power with respect to the load circuit in response to thesecond pulse-width modulation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for showing an arrangement of a class-Damplifying apparatus according to an embodiment of the presentinvention.

FIG. 2 is a timing chart for describing operations of a pulse-widthmodulating circuit employed in the class-D amplifying apparatus of FIG.1.

FIG. 3 is a block diagram for showing an arrangement of the pulse-widthmodulating circuit employed in the class-D amplifying apparatus of FIG.1.

FIG. 4 is a graph for representing contents of conversion functionswhich are used by a converting unit employed in the class-D amplifyingapparatus of FIG. 1.

FIG. 5 is a timing chart for showing a relation between count values andpulses of the pulse-width modulating circuit of FIG. 3.

FIG. 6 is a conceptional drawing for describing effects achieved by thepresent embodiment.

FIG. 7 is a graph for representing contents of conversion functionsaccording to a modification 1 of the present invention.

FIG. 8 is a graph for showing a relation between data and currentamounts of drive currents according to the modification 1.

FIG. 9 is a graph for representing contents of conversion functionsaccording to a modification 1 of the present invention.

FIG. 10 is a timing chart for showing operations of a pulse-widthmodulating circuit according to the modification 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a block diagram for showing an arrangement of a class-Damplifying apparatus 100 according to an embodiment of the presentinvention. As indicated in FIG. 1, the class-D amplifying apparatus 100is equipped with a signal generating apparatus 10 and a drivingapparatus 20. A data series “D Ta” is supplied from an upstreamapparatus to the signal generating apparatus 10. The data series “D Ta”corresponds to a time sequence of Ns-bit data generated at a samplingfrequency “fs”. The signal generating apparatus 10 generates pulse-widthmodulation signals “S1” and “S2” of two systems from the data series “DTa”. It should be understood that when descriptions as to elementsrelated to the pulse-width modulation signal “S1”, and also, elementsrelated to the pulse-width modulation signal “S2” are commonly used inthe below-mentioned descriptions, suffixes “i” (i=1, 2) will be appliedto symbols of these relevant elements in order to omit individualexplanations thereof.

The driving apparatus 20 corresponds to a full-bridge type drivingcircuit which drives a load circuit 30 in a BTL (Bridge Tied Load)system based upon the pulse-width modulation signals “S1” and “S2”. InFIG. 1, such a case that a speaker apparatus is employed as the drivecircuit 30 has been exemplified. The driving apparatus 20 is equippedwith a driving unit 21 and another driving unit 22. Each of the drivingunits 21 and 22 is arranged by switching elements “SWa” and “SWb”, andan inverter circuit “INV”. The switching elements “SWa” and “SWb” areN-channel type field-effect transistors which have been series-connectedbetween power supply lines. An output terminal of the inverter circuitINV is connected to a gate of the switching element SWb. The pulse-widthmodulation signal S1 is supplied to both a gate of the switching elementSWa of the driving unit 21, and an input terminal of the invertercircuit INV of the driving circuit 21. The pulse width modulation signalS2 is supplied to a gate of the switching element SWa of the drivingunit 22, and an input terminal of the inverter circuit INV of thedriving unit 22.

In the above-described arrangement, when a signal level of thepulse-width modulation signal “S1” is set to a high level and a signallevel of the pulse-width modulation signal “S2” is set to a low level, adrive current “I DR” flows through a signal path defined from theswitching element “SWa” of the driving unit 21 via the load circuit 30to the switching element “SWb” of the driving circuit 22. On the otherhand, when a signal level of the pulse-width modulation signal “S1” isset to a low level and a signal level of the pulse-width modulationsignal “S2” is set to a high level, the drive current “I DR” flowsthrough a signal path defined from the switching element “SWa” of thedriving unit 22 via the load circuit 30 to the switching element “SWb”of the driving circuit 21.

As shown in FIG. 1, the signal generating apparatus 10 is equipped witha noise shaping filter 12 and a pulse-width modulating circuit 14. Thenoise shaping filter 12 generates a data series “D Tb” from the dataseries “D Ta” while the noise shaping filter 12 suppresses (namely,noise shaping) a quantize noise in an audio range. The above-describeddata series “D Tb” is a time sequence of such a data “X” whose bitnumber is smaller than the bit number “Ns” of each of the data containedin the data sequence “D Ta”. A sampling frequency “fns” of the dataseries “D Tb” exceeds the sampling frequency “fs” of the data series “DTa”.

The pulse-width modulating circuit 14 modulates each data “X” of thedata series “D Tb” in the pulse-width modulation manner so as togenerate the pulse-width modulation signals “S1” and “S2”. FIG. 2 is atiming chart for explaining operations of the pulse-width modulatingcircuit 14. As represented in FIG. 2, the pulse-width modulation signal“S1” is such a signal that a pulse “P1” has been arranged every unitperiod “T1”. On the other hand, the pulse-width modulation signal “S2”is such a signal that a pulse “P2” has been arranged every unit period“T2”. Both the unit periods “T1” and “T2” are set to a commonly-usedtime length (pulse period) “Tp”. A phase difference between thepulse-width modulation signal S1 and the pulse-width modulation signalS2 is 180 degrees. As a consequence, the respective pulses “P2” of thepulse-width modulation signal S2 are positioned between the adjacentpulses “P1” which are located before and after one pulse “P2” within thepulse-width modulation signal S1.

FIG. 3 is a block diagram for exemplifying a concrete arrangement of theabove-described pulse-width modulating circuit 14. As shown in FIG. 3,the pulse-width modulating circuit 14 includes a converting unit 52,signal generating units “54[1]” and “54[2]”, and an adjusting unit 56.As shown in FIG. 2, the respective data “X” (X[1], X[2], X[3], - - - )of the data series “D Tb” are sequentially supplied from the noiseshaping filter 12 every sampling period “Tns” corresponding to thesampling frequency “fns”. The data series “D Tb” is sectioned while aset (will be referred to as “unit series” hereinafter) “U” of 4 piecesof continuous data “X” is defined as a unit, for the sake ofconvenience.

The signal generating unit 54[1] of FIG. 3 generates the pulse-widthmodulation signal S1, and the signal generating unit 54[2] thereofgenerates the pulse-width modulation signal S2. One pulse P1 of thepulse-width modulation signal S1, and one pulse P2 of the pulse-widthmodulation signal S2 are defined in response to 4 pieces of data “X”(for example, X[1] to X[4]) of one unit series “U”. More preciselyspeaking, the signal generating unit 54[1] sets a time point of a frontedge of the pulse P1 in response to first data “X” (X[1], X[5], - - - )of the unit series “U”, and also the signal generating unit 54 [1.] setsa time point of a rear edge of the pulse P1 in response to second data“X” (X[2], X[6], - - - ) of the unit series “U”. Further, the signalgenerating unit 54[2] sets a time point of a front edge of the pulse P2in response to third data “X” (X[3], X[7], - - - ) of the unit series“U”, and also, the signal generating unit 54[2] sets a time point of arear edge of the pulse P2 in response to fourth data “X” (X[4],X[8], - - - ) of the unit series “U”.

The converting unit 52 of FIG. 3 generates pulse definition data (“DON1”, “D OFF1”, “D ON2”, “D OFF2”) which define both the pulses “P1” and“P2” based upon 4 pieces of the data “X” within the unit series “U”. Apulse definition data “D ONi” corresponds to data which defines a timepoint of a front edge (namely, rising edge) of a pulse “Pi”, and a pulsedefinition data “D OFFi” corresponds to data which defines a time pointof a rear edge (namely, falling edge) of the pulse “Pi”.

In order to generate the pulse definition data (“D NO1”, “D OFF1”, “DON2”, “D OFF2”), 2 sorts of conversion functions “F1” and “F2” are used.FIG. 4 is a graph for exemplifying contents of the conversion functions“F1” and “F2”. As indicated in FIG. 4, the conversion function “F1” isdefined as follows: That is, when the numeral value of the data “X” is aminimum value (−x), the function value “F1(X)” becomes zero, and thefunction value “F1(X)” is linearly increased in connection with anincrease of the numeral value of the data “X”; and when the numeralvalue of the data “X” is a maximum value “(x)”, the function value“F1(X)” becomes a predetermined value “p”. On the other hand, theconversion function “F2” is defined as follows: That is, when thenumeral value of the data “X” is the minimum value (−x), the functionvalue “F2(X)” becomes the predetermined value “p”, and the functionvalue “F2(X)” is linearly decreased in connection with the increase ofthe numeral value of the data “X”; and when the numeral value of thedata “X” is the maximum value “(x)”, the function value “F2(X)” becomeszero.

The converting unit 52 of FIG. 3 sets such a numeral value (P−F1(X[1])to the pulse definition data “D ON1”, which is obtained by subtracting afunction value “F1(X[1])” where the first data “X[1]” of the unit series“U” has been substituted for the conversion function “F1” from apredetermined value “p”, and also, the converting unit 52 sets such anadded value (p+F1(X[2])) to the pulse definition data “D OFF1”, which isobtained by adding another function value “F1(X[2])” where the seconddata “X[2]” of the unit series “U” has been substituted for theconversion function “F1” to the predetermined value “p”. Further, theconverting unit 52 sets such a numeral value (p−F2(X[3])) to the pulsedefinition data “DON2”, which is obtained by subtracting a functionvalue “F2 (X[3])” where the third data “X[3]” of the unit series “U” hasbeen substituted for the conversion function “F2” from the predeterminedvalue “p”, and also, the converting unit 52 sets such an added value(p+F2 (X[4])) to the pulse definition data “DOFF2”, which is obtained byadding another function value “F2(X[4])” where the fourth data “X[4]” ofthe unit series “U” has been substituted for the conversion function“F2” to the predetermined value “p”. It should also be noted thatalthough the unit series “U” of the data X[1] to X[4] have beenexemplified in the above description, the pulse definition data (D ON1”,“D OFF1” “D ON2”, “D OFF2”) may be sequentially generated in accordancewith a similar rule with respect to other unit series “U” (for example,unit series “U” which are constituted by data X[5] to X[8]).

The signal generating unit 54 [i] is constituted by holding units 62A[i]and 62B[i], a counting unit 64[i], comparing units 66A[i] and 66B[i],and a waveform generating unit 68[i]. Signals “TRGi” are supplied to theholding units 62A[i] and 62B[i]. As indicated in FIG. 2, a signal TRG1and another signal TRG2 correspond to such signals having time periodsequal to the pulse period “TP”, the phase difference of which is 180degrees. In other words, the signal TRG1 falls at a starting point(endpoint) of each unit time period “T1”, and the signal TRG2 falls at astarting point (end point) of each unit time period “T2”. As shown inFIG. 2, the holding unit 62A[i] corresponds to a circuit (latch circuit)which holds and outputs the pulse definition data “D ONi” generated bythe converting unit 52 at a falling time point of the signal “TRGi”.Similarly, the holding unit 62B[i] corresponds to a circuit (latchcircuit) which holds and outputs the pulse definition data “D OFFi”generated by the converting unit 52 at a falling time point of thesignal “TRGi”.

The counting unit 64[i] of FIG. 3 is a counter which generates a countvalue “Ci”. As indicated in FIG. 2, the count value “Ci” is initializedto become zero at a starting point of a unit time period “Ti”, islinearly increased in connection with a time elapse, and then, reaches apredetermined value “2p” at an end point of the unit time period “Ti”.As a consequence, the count value “Ci” represents a waveform having asaw-tooth shape, the time period of which is equal to a pulse period“TP”.

FIG. 5 is a timing chart for representing a relation between thecount-value “Ci” and a pulse-width modulation signal “Si” within asingle unit time period “Ti”. A comparing unit 66A[i] compares pulsedefinition data “D ONi” which is outputted from a holding unit 62A[i]with the count value “Ci” which is outputted from the counting unit 64[i], and as shown in FIG. 5, the comparing unit 66A[i] outputs a setsignal “P SET” at a time point when the count value Ci exceeds thenumeral value of the pulse definition data “D ONi”. On the other hand, acomparing unit 66B[i] compares pulse definition data “D OFFi” which isoutputted from a holding unit 62B[i] with the count value “Ci” which isoutputted from the counting unit 64[i], and as shown in FIG. 5, thecomparing unit 66B[i] outputs a reset signal “P RES” at a time pointwhen the count value “Ci” exceeds the numeral value of the pulsedefinition data “D OFF”.

The waveform generating unit 68 [i] of FIG. 3 changes the signal levelof the pulse-width modulation signal “Si” from a low level to a highlevel (namely, forms a front edge of pulse “Pi”) at the time point whenthe set signal “P SET” is outputted from the comparing unit 66A[i], andchanges the signal level of the pulse-width modulation signal “Si” fromthe high level to the low level (namely, forms a rear edge of pulse“Pi”) at the time point when the reset signal “P RET” is outputted fromthe comparing unit 66B[i]. As a consequence, as shown in FIG. 5, thepulse “Pi” has such a shape that a portion preceding from a time point(namely, center point) “tc”, which divides the unit time period “Ti” by½, by a time length in response to the pulse definition data “DONi” isdefined as the front edge, whereas a portion in which a time lengthresponding to the pulse definition data “D OFFi” has elapsed from thecenter point “tc” is defined as a rear edge.

The larger the numeral value of the data “X” becomes, the smaller thenumeral value (p−F1(X)) of the pulse definition data “D ON1” becomes. Asa result, the larger the numeral value of the data “X” becomes, thefurther the front edge of the pulse P1 is moved to the former timepoint. Also, the larger the numeral value of the data “X” becomes, thelarger the numeral value (p+F1(X)) of the pulse definition data “D OFF1”becomes. As a result, the larger the numeral value of the data “X” isincreased, the later the rear edge of the pulse P1 is moved to the latertime point. In other words, the larger the numeral value of the first,or second data “X” of the unit series “U” becomes, the wider the pulsewidth of the pulse “P1” in the pulse-width modulation signal “S1”becomes. For instance, in such a case that the numeral values of thedata X[1] and the data X[2] of the unit series “U” are maximum values(x), the function values F1(X[1]) and F1(X[2]) become the predeterminedvalue “p” (refer to FIG. 4), the numeral value (p−F1(X[1])) of the pulsedefinition data “D ON1” is set to zero, and also, the numeral value(p+F1(X[2])) of the pulse definition data “D OFF2” is set to thepredetermined value “2p”. As a consequence, a duty ratio of thepulse-width modulation signal “S1” within the unit time period “T1”becomes 100%.

On the other hand, the lager the numeral value of the data “X” becomes,the larger the numeral value (p−F2(X)) of the pulse definition data “DON2” becomes. As a result, the larger the numeral value of the data “X”becomes, the further the front edge of the pulse “P2” is moved to thedelayed time point. Also, the larger the numeral value of the data “X”becomes, the smaller the numeral value (p+F2(X)) of the pulse definitiondata “D OFF2” becomes. As a result, the larger the numeral value of thedata “X” becomes, the further the rear edge of the pulse “P2” is movedto the former time point. In other words, the larger the numeral valueof the third, or fourth data “X” of the unit series “U” becomes, thenarrower the pulse width of the pulse “P2” in the pulse-width modulationsignal “S2” becomes. For instance, in such a case that the numeralvalues of the data X[3] and the data X[4] of the unit series “U” aremaximum values (x), the function values F2(X[3]) and F2(X[4]) becomezero (refer to FIG. 4), so that any of the numeral value (p−F1(X[3])) ofthe pulse definition data “D ON2” and also, the numeral value(p+F2(X[4])) of the pulse definition data “D OFF2” is set to thepredetermined value “p”. As a consequence, a duty ratio of thepulse-width modulation signal “S2” within the unit time period “T2”becomes 0%.

Although it is possible to state that the large/small relationshipbetween the numeral values of the data “X”, and the short/longrelationship between the pulse widths with respect to the pulse “P1” and“P2” have the opposite sense, for example, in such a case that thenumeral value of the data “X” within the unit series “U” is largelychanged, there are some possibilities that the signal levels of both thepulses “P1” and “P2” are transferred to high levels (in other words,pulse “P1” is overlapped with pulse “P2). For instance, in such a casethat the data “X[2]” of the unit series “U” has a sufficiently largenumeral value and the data “X[3] ” thereof has a sufficiently smallnumeral value, both the pulse widths of the pulses “P1” and “P2” areincreased, and then, the increased pulse widths are overlapped with eachother. The adjusting unit 56 of FIG. 3 is provided to avoid theabove-explained overlapping phenomenon of the pulses P1 and P2. In otherwords, when the pulse “P1” is overlapped with the pulse “P2”, theadjusting unit 56 changes logic levels of the pulse-width modulationsignals “S1” and “S2”. For example, the adjusting unit 56 maintains oneof the pulses “P1” and “P2” in a high level, and further, changes thesignal level of the other pulse into a low level in a forcible manner.With employment of the above-described arrangement, it is possible toeliminate such a possibility that both the switching element “SWa” ofthe driving unit 21 and the switching element “SWa” of the driving unit22 are conducted at the same time. As a result, the load circuit 30 canbe properly driven in response to the respective data “X” of the dataseries “D Tb”.

As previously described, the pulse-width modulation signal “S1” becomessuch a signal that the pulse “P1” where two pieces of the front halfdata “X” within the unit series “U” have been modulated in thepulse-width modulating manner are arranged every unit time period “T1”.Further, the pulse-width modulation signal “S2” becomes such a signalthat the pulse “P2” where two pieces of the rear half data “X” withinthe unit series “U” have been modulated in the pulse-width modulationmanner are arranged every unit time period “T2”. As a consequence, theresolution (namely, stepping width of current amount of drive current “IDR”) as to the pulse-width modulation signals “S1” and “S2” can beimproved, as compared with that of a circuit arrangement in which apulse width of a single pulse of a pulse-width modulation signal iscontrolled by one piece of data.

Further, as previously described, 4 pieces of the data “X” within theunit series “U” are modulated in the pulse-width modulation mannerwithin the pulse period “TP”. As a result, as shown in FIG. 2, the pulseperiod “TP” is set to such a time length equal to 4 time periods of thesampling period “Tns”. In other words, the sampling frequency “fns” canbe set to be 4 times higher than such a frequency (will be referred toas “carrier frequency” hereinafter) “fp” which corresponds to the pulseperiod “TP” of the pulse-width modulation signal “S1”, or “S2”. Both apart (A) and another part (B) of FIG. 6 are conceptional diagrams forindicating a relation between a quantize noise “CN” and a signalcomponent “CS”, which are represented by the data series “D Tb”outputted from the noise shaping filter 12. The part (A) of FIG. 6supposes such a conventional structure that the sampling frequency “fns”has been set to the same frequency as the carrier frequency “fp”,whereas the part (B) of FIG. 6 supposes such a structure of the presentembodiment that the sampling frequency “fn” has been set to be 4 timeshigher than the carrier frequency “fp”. As shown in FIG. 6, in thepresent embodiment, since the sampling frequency “fns” is increased, arange where the quantize noise “CN” is distributed is enlarged, and onthe other hand, a strength of the quantize noise “CN” is reduced byapproximately ¼, as compared with the case shown in the part (A) of FIG.6. The above-described strength of the quantize noise “CN” implies,especially, such a strength of the quantize noise “CN” which issuperimposed with a range where the signal component “CS” isdistributed. As a consequence, it is possible that a dynamic range of acurrent amount of the drive current “I DR” can be sufficiently secured(S/N ratio of drive current “I DR” can be improved).

It should also be noted that the above-described respective embodimentsmay be modified in a various manner. Concrete modifications will beexemplified in the below-mentioned descriptions. Alternatively, thebelow-mentioned modifications may be combined with each other.

(1) Modification 1

The relationship between the numeral values of the respective data “X”of the data series “ID Tb1”, and the function values F1(X) and F2(X) isnot limited only to the exemplified relationship of FIG. 4. For example,as indicated in FIG. 7, the conversion functions “F1” and “F2” may bealternatively defined in such a manner that when the numeral value ofthe data “X” becomes smaller than a predetermined value (−x0), thefunction value “F1(X)” becomes zero, whereas when the numeral value ofthe data “X” exceeds the predetermined value (x0), the function valueF2(X) becomes zero. In the embodiment of FIG. 7, when the numeral valueof the data “X” becomes smaller than the predetermined value (−x0), thepulse width of the pulse P1 becomes zero, whereas when the numeral valueof the data “X” exceeds the predetermined value (x0), the pulse width ofthe pulse P2 becomes zero. As a consequence, a current amount of a drivecurrent “I DR” flowing through the load circuit 30 may be suppressed. Itshould also be understood that in the embodiment of FIG. 7, asexemplified in FIG. 8, a relationship between the numeral value of thedata “X” and the current amount (level) of the drive current “I DR” doesnot constitute a straight line. As a consequence, since the conversionfunctions “F1” and “F2” are defined as represented in FIG. 9, such anarrangement that the current amount of the drive current “I DR” islinearly changed in response to the numeral value of the data “X” mayalso be suitably employed.

(2) Modification 2

In the above-described embodiments, the signal generating apparatus 10generates both the pulse-width modulation signals “S1” and “S2” of thetwo systems. Alternatively, a total number of pulse-width modulationsignals “S” may be properly changed. FIG. 10 is a timing chart forexplaining operations for generating pulse-width modulation signals “S1”to “S3” of 3 systems by the signal generating apparatus 10. A unitseries “U” obtained by segmenting the data series “D Tb” contains 6pieces of data “X” (X[1] to X[6]). The pule-width modulation signals“S1” and “S2” may be generated in response to the data X[1] to X[4]within the unit series “U” in a similar manner to the above-describedembodiment. On the other hand, as to a pulse “P3” of the pulse-widthmodulation signal “S3”, a time point of a front edge thereof is set inresponse to the fifth data X[5], and a time point of a rear edge thereofis set in response to the sixth data X[6] within the unit series “U”. Inaccordance with the above-described arrangement, the sampling period“Tns” may be furthermore shortened with respect to the pulse period“TP”.

1. A signal generating apparatus comprising: a data generator whichgenerates a data series in which a plurality of data containing firstdata, second data, third data, and fourth data are arranged at apredetermined sampling period; a first signal generator which generatesa first pulse-width modulation signal in which a pulse is arranged in apulse period longer than the predetermined sampling period, a time pointof a front edge of the pulse being set in response to the first data,and a time point of a rear edge of the pulse being set in response tothe second data; and a second signal generator which generates a secondpulse-width modulation signal in which a pulse is arranged between theadjacent pulses of the first pulse-width modulation signal, a time pointof a front edge of the pulse of the second pulse-width modulation signalbeen set in response to the third data, and a time point of a rear edgeof the pulse of the second pulse-width modulation signal been set inresponse to the fourth data.
 2. The signal generating apparatusaccording claim 1, wherein the first signal generator generates thefirst pulse-width modulation signal in such a manner that the larger thenumeral value of the first data, or the second data becomes, the widerthe pulse width thereof becomes; and the second signal generatorgenerates the second pulse-width modulation signal in such a manner thatthe larger the numeral value of the third data, or the fourth databecome, the narrower the pulse width thereof becomes.
 3. The signalgenerating apparatus according claim 1 further comprising: an adjustingunit which changes a logic level of the first pulse-width modulationsignal and a logic level of the second pulse-width modulation signal tobe different from each other in such a case that the pulse of the firstpulse-width modulation signal is overlapped with the pulse of the secondpulse-width modulation signal.
 4. The signal generating apparatusaccording to claim 1 further comprising: a converting unit whichconverts the first data into a first pulse definition data, converts thesecond data into a second pulse definition data, converts the third datainto a third pulse definition data and convert the fourth data into afourth pulse definition data; wherein the first signal generatorincludes: a first holding unit which holds the first pulse definitiondata; a second holding unit which holds the second pulse definitiondata; a first counter which generates first count values representing awaveform having a saw-tooth shape; a first comparator which compares thefirst pulse definition data with the first count values; a secondcomparator which compares the second pulse definition data with thefirst count values; and a first waveform generating unit which generatesthe first pulse-width modulation signal based on the comparison resultsof the first and second comparators, and wherein the second signalgenerator includes: a third holding unit which holds the third pulsedefinition data; a fourth holding unit which holds the fourth pulsedefinition data; a second counter which generates second count valuesrepresenting a waveform having a saw-tooth shape; a third comparatorwhich compares the third pulse definition data with the second countvalues; a fourth comparator which compares the fourth pulse definitiondata with the second count values; and a second waveform generating unitwhich generates the second pulse-width modulation signal based on thecomparison results of the third and fourth comparators.
 5. A class-Damplifying apparatus comprising: a data generator which generates a dataseries in which a plurality of data containing first data, second data,third data, and fourth data are arranged at a predetermined samplingperiod; a first signal generator which generates a first pulse-widthmodulation signal in which a pulse is arranged in a pulse period longerthan the predetermined sampling period, a time point of a front edge ofthe pulse being set in response to the first data, and a time point of arear edge of the pulse being set in response to the second data; asecond signal generator which generates a second pulse-width modulationsignal in which a pulse is arranged between the adjacent pulses of thefirst pulse-width modulation signal, a time point of a front edge of thepulse of the second pulse-width modulation signal been set in responseto the third data, and a time point of a rear edge of the pulse of thesecond pulse-width modulation signal been set in response to the fourthdata; a first driver for controlling a supply of electric power withrespect to a load circuit in response to the first pulse-widthmodulation signal; and a second driver for controlling the supply of theelectric power with respect to the load circuit in response to thesecond pulse-width modulation signal.